Piezoelectric materials, especially piezoelectric ceramics such as PZT (lead zirocnate titanate) are the most widely known materials used for the manufacture of actuators, sensors and transducers among all ferroelectric-type materials (poled ferroelectric material is piezoelectric material). Recently, however, other materials have begun to be developed for use as actuators, sensors, and transducers, which have some beneficial properties not found in piezoelectric materials. One of these materials is an antiferroelectric material, such as antiferroelectric ceramics found in PZT or PZST (lead zirconate stannate titanate) systems.
Antiferroelectric materials have anti-parallel dipoles which can be transferred into a parallel state (ferroelectric state) under an applied field, thus double hysteresis loops will appear on their P (polarization)-E (electric field) curve. A large strain (about 0.4% to 0.8%) accompanies an antiferroelectric-ferroelectric phase switching, which is four to eight times higher than that which can be reached in most piezoelectric ceramics (usually less than 0.1%). Additionally, different types of antiferroelectric materials may be used to generate different strain levels within this approximate 0.4% to 0.8% range.
Antiferroelectric materials are uni-directional actuators, that is, no matter the direction of the applied field, the material will always expand. Also, the mentioned large strain jump around the phase switching field, is a “digital” type actuation characteristic, which makes antiferroelectric materials suitable for ON/OFF actuation applications such as a pump. Antiferroelectric materials also have a minus Poisson's ratio. This means that, during the antiferroelectric/ferroelectric phase switching, the material will expand in all directions. Still a further characteristic of antiferroelectric material is that it does not require a poling operation as needed by piezoelectric materials.
Thus, due to their physical characteristics, both piezoelectric and antiferroelectric materials are considered useful in the fabrication of structures which morph from one position to another. Therefore, both materials are used in unimorph macro-world sized designs, i.e., where a single morphing film layer is used. Piezoelectric materials are also used in macro-world sized bimorph structures. However, antiferroelectric/antiferroelectric bimorph macro-world sized structures are not available due in part to the described nature of antiferroelectric material.
Additionally, while it is known bimorphs can provide as much as twice the voltage or charge of unimorphs under the same mechanical loading, and can provide twice the displacement of unimorphs under the same electric driving condition, only piezoelectric and antiferroelectric unimorphs are now used in microelectro-mechanical dimensioned (MEMS) systems. MEMS-type bimorphs have not been developed due at least in part to manufacturing obstacles related to building two piezoelectric or antiferroelectric thin or thick film layers together, or in combination with each other. A further obstacle is developing a system which is able to pole the piezoelectric layers with suitable directions, and to make antiferroelectric/antiferroelectric bimorphs move along two directions.
As mentioned, piezoelectric bimorph and unimorph bending devices have been used in macro-world sized designs. These devices, made from bulk ceramics, can be fabricated as a cantilever, diaphragm or other structure to then be used as sensors, actuators, and transducers. Shown in FIGS. 1A-1C are cantilever bimorph and unimorph macro devices.
Piezoelectric bimorph devices 10 of FIGS. 1A and 12 of FIG. 1B each consist of two ceramic plates bonded together in two types of connections often used in bimorph fabrication. In FIG. 1A, a series or anti-parallel connection 14 is used in which the two piezoelectric sheets 16, 18 with opposite polarization directions are bonded together, and the electrical connection 20 is applied by electrodes 22, 24 across the total thickness. FIG. 1B is a parallel connection 26, in which two piezoelectric sheets 28, 30 with the same polarization direction, are bonded together, and an electrical connection 32 is applied between an intermediate electrode 34 and the top/bottom electrodes 36, 38.
The unimorph actuator 40 of FIG. 1C consists of either one piezoelectric or antiferroelectric sheet 42 and one passive metal plate 44, such as metal plate bonded together. Unimorph 40 is driven by electrical connection 46, which is in contact with metal plate 44 and electrode 48.
Because bimorphs consist of two active layers and unimorphs consist of one active layer and one passive layer, bimorphs are more efficient devices than unimorphs. For example, in sensor applications under the same mechanical loading, the induced charge (for a parallel-connected piezoelectric bimorph) or the induced voltage (for a series-connected piezoelectric bimorph) doubles that of a unimorph with the same dimensions. Therefore, in actuator applications, under the same driving condition, the tip displacement of a bimorph will double that of a unimorph.
As it is difficult to make bulk piezoelectric ceramic sheets with a thickness less than 100 μm, the thickness of the piezoelectric sheets used in bimorphs and unimorphs are usually from several hundreds μm to several mm.
One reported fabrication of MEMS thin and thick film bimorphs is by depositing piezoelectric thin or thick films on both sides of a metal plate or foil through sputtering or hydrothermal growth processes. The thickness of the metal plate or foil is commonly thicker than 25 μm. Due to the size of the metal plate or foil, a passive or dampening effect must be taken into account with such “triple layer” bimorphs. Due to the dampening effect, the bimorphs created by this process, are not as efficient as “true” bimorphs made by direct bonding of two films or layers together.
The state of the art does not provide an efficient high-yield process for the manufacture of true bimorph-based structures in the MEMS scale, where the layers are piezoelectric/piezoelectric, antiferroelectric/antiferroelectric or antiferroelectric/-piezoelectric combinations. Also, usable antiferroelectric/antiferroelectric macro-sized bimorph structures are not now available.
While MEMS thick film bimorph devices, with a piezoelectric film thickness between 10 to 100 μm, are desirable as it would provide a larger force and broader working frequency range than MEMS thin film bimorphs, MEMS thick film bimorphs in this range have also not been produced. This is due at least in part to current film production processes which fail to provide an efficient manner of producing piezoelectric films within this range, and therefore no consideration has been given to making two-layer thick films together in this range.